1. Laser Micro/Nano Fabrication LaboratorySchool of Mechanical EngineeringBeijing Institute of Technology Beijing 100081 P. R. China
2. School of Optics and PhotonicsBeijing Institute of Technology Beijing 100081 P. R. China
3. Tianjin Navigation Instruments Research Institute Tianjin 300131 P. R. China
4. Department of PhysicsCapital Normal UniversityBeijing Key Laboratory of Metamaterials and DevicesKey Laboratory of Terahertz OptoelectronicsMinistry of Education, and Beijing Advanced InnovationCenter for Imaging Technology Beijing 100048 P. R. China
5. Department of Electrical EngineeringUniversity of Nebraska‐Lincoln Lincoln NE 68588‐0511 USA